Why doesn’t my bike have an alternator light?

Alternator: A small electrical generator driven by the engine used to maintain the level of charge in a vehicle’s battery. 

If you’ve recently moved over to a bike from a car, you might’ve noticed your two wheeled friend is missing an alternator warning (or battery) light telling you everything’s ok with your charging system. Why is this?

motorcycle_bmw_f650

It’s not that manufacturers couldn’t fit one in, it’s just that in engineering terms it’s not deemed necessary. And the reason for that decision is that unlike in cars, most motorcycles don’t use rubber drive belts in their charging systems.

With heavy electrical requirements but plenty of room around their engines, cars use bulky separate alternators connected by rubber V, ribbed or toothed belts to an engine pulley in order to turn them and create electricity. This makes checking, service and replacement of all components straightforward.

But while the alternators themselves are quite robust and long lasting, the rubber drive belts which turn them are not.

car-alternator
Car alternator (arrowed) connected to the engine by a rubber belt

You don’t need to be a mechanic to know that rubber belts are a pain – you’ve probably seen the shredded evidence on the side of the road frequently enough. Belts stretch, crack, fray and jump. Then they break. And because of that fragility, alternator warning lights are considered wise on cars as slipping or broken belts are likely fail scenarios in vehicle charging systems.

With less demanding electrical needs and space at a premium, bikes don’t need such big alternators as cars do, so their more compact components can be cleverly built-in to the engines themselves.

Systems vary, but a typical modern bike incorporates its generating equipment directly on to one end of the crankshaft which is then hidden behind an anonymous cover. You’d have to take the engine apart to find it, like this:

motorcycle-alternator
Motorcycle alternator parts (arrowed) incorporated into the engine internals. Credit f650.com

Looking carefully at this partially dismantled BMW F650, you can identify the two main components of an alternator which have been separated for service – the magnetised rotor and the wire-wound stator in the mechanic’s hand. But what you won’t see are any nasty rubber belts!

Being bolted directly to the engine means (everything else being equal) the drive system for a typical bike charging system is simpler and more reliable than a car’s belt driven equivalent: a motorcycle’s alternator is always turning to create electricity whenever the engine is running.

So with no belts to break or slip, there’s no need for that warning light you thought you were missing. It’s as simple as that.

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